1. Field of the Invention
This invention relates to velocity of flow rate measurement of fluid flow. In particular, the invention involves the use of photobleaching of fluorescence dye to measure the fluid flow velocity, including direction, of the fluids in devices. The invention can be employed in a wide variety of applications including, but not limited to, the processes, apparatuses, devices and instrument in scales of conventional, millimeter, micro and nanometer. This invention is non-intrusive and capable of measuring flow velocity inline, point measurement, two dimensional and three dimensional flow fields with high temporal and spatial resolution.
2. Description of Related Art
The measurement of fluid velocity by fluorescence photobleaching is known in the art. Reviews of the measurement of fluid velocity with fluorescence photobleaching are presented in the following references:    C. A. Monnig and J. W. Jorgenson, Anal. Chem., 1991, 63, 802-807;    A. W. Moore and J. W. Jorgenson, Anal. Chem., 1993, 65, 3550-3560;    B. P. Mosier, J. I. Molho and J. G. Santiago, Exp. Fluids, 2002, 33, 545-554;    K. F. Schrum, J. M. Lancaster, S. E. Johnston and S. D. Gilman, Anal. Chem., 2000, 72, 4317-4321;    J. L. Pittman, K. F. Schrum and S. D. Gilman, Analyst, 2001, 126, 1240-1247;    J. L. Pittman, C. S. Henry and S. D. Gilman, Anal. Chem., 2003, 75, 361-370.    H. E. Fiedler and G. R. Wang, Deutsches Patent, 19838344.4, 1998, Germany;    J. Ricka, Exp. Fluids, 1987, 5, 381-384;    J. White and E. Stelzer, Trends Cell Biol., 1999, 9, 61-65;    B. Storrie, R. Pepperkok, E. Stelzer and T. E. Kreis, J. Cell Science, 1994, 107, 1309-1319.    G. R. Wang Laser-induced fluorescence photobleaching anemometer for microfluidic devices. Lab on a Chip, 2005, 5, 450-456.
For instance, fluorescence recovery after photobleaching (FRAP), which can be used to measure very low flow velocity near the region of Brownian motion, requires two laser beams at the detection point. One is high laser beam intensity to cause the photobleaching and the other one is used to measure the recovered fluorescence intensity due to molecular diffusion for a long time period. The new method can measure the flow velocity instantaneously with only one beam and the velocity range measured is higher than the molecular diffusion. The two points based method in Pittman et al. (2003) also requires two laser beams and the first one bleaches the fluorescence and the second one has to be in the downstream of the first one in a distance to measure the bleached fluorescence signal. This method can only measure the bulk flow velocity and the temporal resolution is limited, since it has to wait for the bleached dye plug to translate from the first laser beam to the second one. Photobleached Fluorescence Visualization in Mosier (2002) requires camera to monitor images of flow field at different downstream positions to calculation flow velocity. Also a diffusion model is required. Rick (1987) published only qualitative method to visualize flow velocity, where the optical setup, detector and flow used for calibration were different from the real detector and flow. This make the quantitative measurement impossible, since the calibration relationship between velocity and fluorescence intensity is different from that in real flow. Also the calibration was linear. All these method cannot measure spatial velocity distribution, neither can they measure velocity vector, i.e. transverse velocity components.
The present invented method is a different way from aforementioned methods to measure flow velocity, although it also use photobleaching. In the invented method, the optical setup, detector and the flow are all the same to be able to carry out precise quantitative measurement. The calibration relationship in current method is not necessarily linear, but can also be a polynomial or exponential relation. Since the invented method is based on a single point measurement, it can measure velocity distribution in the transverse direction with high spatial resolution; the velocity is directly measured with the calibration relationship between flow velocity and fluorescence intensity, and thus the invented method has high temporal resolution and can be used for inline or online flow velocity or flow rate monitoring. Since molecular tracer dye is used for the invented method, the spatial resolution is high and can be used to devices of Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS) compared with Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA) and Ultrasound Velocimetry. Using evanescent wave guide, the invented method can also measure flow velocity in near solid wall region.